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Kenji Watanabe

Publications and source records attributed to Kenji Watanabe.

At least 145 records · Page 8Linked to original sources
arXiv2026

Third-order nonlinear transport in a percolative two-dimensional superconductor

Percolative superconductivity frequently arises in two-dimensional van der Waals materials due to reduced dimensionality, enhanced quantum fluctuations, and complex electron-phonon interactions, providing a unique platform where normal electrons coexist with Cooper pairs. We report the observation of substantial third-order nonlinear transport in a trilayer $1T^\prime$-MoTe$_2$ superconductor within its percolative transition regime. The third-harmonic longitudinal voltage ($V_{\|}^{3ω}$) exhibits a clear cubic dependence on excitation current below a threshold, with both its magnitude and nonlinear coefficient strongly correlated with the superconducting state. This nonlinear response is semiquantitatively captured by the superconducting fluctuation within the time-dependent Ginzburg-Landau theory, where nonlinear transport arises due to fluctuating Cooper pairs. Our results demonstrate that third-order nonlinear transport serves as a sensitive probe of superconducting transitions in percolative systems and establish a foundation for exploring higher-order transport phenomena in strongly correlated systems.

cond-mat.mes-hall↗
arXiv2026

Large Thermopower with Sign-Alternating Quantum Oscillations in Insulating Monolayer WTe2

The detection of Landau-level-like energy structures near the chemical potential of an insulator is essential to the search for a class of correlated electronic matter hosting charge-neutral fermions and Fermi surfaces, a long-proposed concept that remains elusive experimentally. Here we introduce and demonstrate that the magneto-thermoelectric response of a quantum insulator can reveal critical information not available via other approaches. We report the observation of large thermopower together with quantum oscillations (QOs) in the hole-doped insulating state of monolayer tungsten ditelluride (WTe2) in magnetic fields. The measured low temperature magneto-thermopower exceeds k_B/e by more than an order of magnitude, where k_B is the Boltzmann constant and e the elementary charge. This large thermopower is a characteristic of an insulating state, consistent with high resistivity. However, as the magnetic field is swept, QOs develop in the thermopower, which remarkably undergoes sign-changes that mimic the quantum characteristic of metals due to Landau quantization. The resistivity of the sample is orders of magnitude larger than the value expected from the QOs. The sign-change in the thermoelectric response directly implies the presence of a field-induced Landau-level-like structure at the chemical potential of the insulator. Neither the large thermopower nor the sign-changes can be induced by the metallic gate nearby. Our results demonstrate a new dilemma for investigating low energy excitations in correlated materials featuring mixed quantum characteristics of metals and insulators.

cond-mat.str-el↗
arXiv2026

Same-spin Andreev reflections in the quantum Hall regime: the role of loss

The interfaces of superconductors and topological materials hold promise for realizing exotic states and excitations. An important example is provided by the chiral Andreev edge states (CAES), which are formed at interfaces between quantum Hall (QH) states and superconductors (SC). CAES combine electron and hole amplitudes which are hybridized via Andreev reflections. This study explores the spin properties of the CAES through selective spin filtering of the QH edge channels. We find robust evidence of spin-flips accompanying the Andreev processes: electrons can be reflected from the superconductor as holes in the same spin channel. We demonstrate that the distribution of the reflection probabilities is exponential and then use random matrix theory to account for this observation. Finally, we observe Andreev reflections in the spin-polarized ν = 1 case, which is enabled by particle loss. Our findings shed light on the mechanism underlying Andreev reflections of spin-polarized chiral states. They also demonstrate the importance of considering non-Hermiticity when constructing topological superconductors in hybrid materials.

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arXiv2026

Scanning Gate Microscopy Modulation of Supercurrent in Graphene Josephson Junctions

Graphene Josephson junctions represent an excellent platform for quantum technologies, thanks to the combination of high carrier mobility, ballistic transport, and large gate-tunable critical currents, preserved even under quantizing magnetic fields. Investigating the spatial distribution of supercurrent flow could be crucial for elucidating transport mechanisms and advancing the engineering of these devices. In this work, we employ a Scanning Gate Microscope to investigate supercurrent transport in hBN-encapsulated graphene Josephson junctions contacted by Niobium leads. We study the supercurrent modulation as a function of the applied tip voltage bias and tip-to-sample distance, and provide a complete characterization of the tip-induced modulation. Our experimental results are quantitatively consistent with numerical simulations and pave the way towards local mapping and manipulation of gate-tunable superconducting phenomena with unprecedented spatial resolution.

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arXiv2026

Visualizing the Impact of Quenched Disorder on 2D Electron Wigner Solids

Electron Wigner solids (WSs)1-12 provide an ideal system for understanding the competing effects of electron-electron and electron-disorder interactions, a central unsolved problem in condensed matter physics. Progress in this topic has been limited by a lack of single-defect-resolved experimental measurements as well as accurate theoretical tools to enable realistic experiment/theory comparison. Here we overcome these limitations by combining atomically-resolved scanning tunneling microscopy (STM) with neural-quantum-state quantum Monte Carlo (NQS-QMC) simulation of disordered 2D electron WSs to discover new disorder-induced physical regimes of correlated electron behavior. STM was used to image the electron density ($n_e$) dependent evolution of electron WSs in gate-tunable bilayer MoSe2 devices with varying long-range ($n_\mathrm{LR}$) and short-range ($n_\mathrm{SR}$) disorder densities. These images were compared to NQS-QMC simulations using realistic disorder maps extracted from experiment, thus allowing the roles of different disorder types to be disentangled. We identify two distinct physical regimes for disordered electron WSs that depend on $n_\mathrm{SR}$. For $n_\mathrm{SR} \lesssim n_e$ the WS behavior is dominated by long-range disorder and features extensive mixed solid-liquid phases, a new type of local re-entrant melting/crystallization, and prominent Friedel oscillations. In contrast, when $n_\mathrm{SR} \gg n_e$ these features are suppressed and a more robust amorphous WS phase emerges that persists to higher ne, highlighting the importance of short-range disorder in this regime. Our work establishes a powerful framework for studying disordered quantum solids via a combined experimental-theoretical approach.

cond-mat.str-el↗
arXiv2026

Space-Charge-Limited van der Waals Spin Transistor

Integrating semiconducting and magnetic materials could combine transistor-like operation with nonvolatility and enable architectures such as logic-in-memory. Here, we employ correlated electrical transport and scanning nitrogen-vacancy (NV) center magnetic imaging to elucidate a spin transistor concept that amalgamates vertical and lateral hopping transport inside a 2D antiferromagnetic semiconductor, mechanistically distinct from vertical tunneling devices. Our device, based on a monolayer-bilayer junction in CrSBr, displays giant, gate-tunable magnetoresistance driven by the dual action of electrostatic doping on space-charge-limited lateral conduction and interlayer exchange coupling. Moreover, we visualize a field-trainable, layer-sharing effect that selects between coherent or domain-wall reversal at the spin-flip transition, enabling multilevel, memristive conductance states. Our layer-dependent space charge mechanism for convergent electrical and magnetic control opens opportunities to address limitations in contemporary computing.

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arXiv2026

Van Hove singularity-driven giant Nernst signal in twisted double bilayer graphene

Twisted graphene layers host van Hove singularities (vHSs), peaks in the electronic density of states, thought to drive exotic phases in moiré materials, but their effect on thermal transport has remained unclear. Here we show that vHSs in twisted double bilayer graphene (tDBLG) generate an unusually large Nernst signal-the transverse voltage produced by a longitudinal temperature gradient in a magnetic field. The pronounced Nernst peaks at the vHSs of the conduction and valence bands of tDBLG are tunable by an electric field with a maximum value of $\sim 40$ $μV K^{-1} T^{-1}$ at $\sim 1$ $K$, which is comparable to the best-known Nernst materials. Our theoretical calculations show that the large enhancement of the Nernst signal arises from the Lifshitz transitions around the vHSs. These findings establish the Nernst effect as a sensitive probe of Fermi-surface topology in moiré materials, and identify a universal thermoelectric signature of van Hove singularities.

cond-mat.mes-hall↗
arXiv2026

Quantum oscillation spectroscopy of Fermi-surface topologies in tetralayer graphene

Quantum oscillations offer a direct probe of Fermi-surface topology and electronic degeneracy, yet disentangling both simultaneously across the Lifshitz transitions of multiband systems has remained an open experimental challenge. Here, we use Shubnikov-de Haas spectroscopy on a high-mobility, dual-gated Bernal-stacked tetralayer graphene (B-4LG) device to quantitatively reconstruct the complete sequence of six distinct Fermi-surface topologies-gully, annular, singly connected, and multiband pockets. The extracted oscillation frequencies determine the extremal momentum-space areas and their spin, valley, and gully-resolved degeneracies, in quantitative agreement with our tight-binding calculations. We further show that a perpendicular magnetic-field, combined with displacement-field lifts the valley degeneracy through an orbital-Zeeman coupling, producing a single-particle valley splitting of several $meV$, far larger than in bilayer or trilayer graphene. Our work demonstrates a framework for tracking Fermi-surface topologies and their flavor degeneracies in multiband quantum materials.

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arXiv2026

Direct visualization of gate-tunable flat bands in twisted double bilayer graphene

The symmetry-broken correlated states in twisted double bilayer graphene (TDBG) can be tuned via several external knobs, including twist angle, displacement field, and carrier density. However, a direct, momentum-resolved characterization of how these parameters reshape the flat-band structure remains limited. In this study, we employ micro focused angle-resolved photoemission spectroscopy to investigate the flat-band dispersion of TDBG at a twist angle of 1.6, systematically varying the displacement field and carrier density via electrostatic gating. We directly observe multiple flat moir'e minibands near charge neutrality, including a flat remote valence band residing below the low-energy flat-band manifold. Furthermore, the dominant Coulomb repulsive energy over the flat- band bandwidth suggests favorable conditions for the emergence of interaction-driven correlated phenomena in TDBG. These findings establish that the formation and evolution of flat bands in TDBG arises from the interplay between the electron filling and the displacement field.

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arXiv2026

Terahertz electrodynamics in a zero-field Wigner crystal

In clean two-dimensional (2D) systems, electrons are expected to self-organize into a regular lattice, a Wigner crystal, when their mutual Coulomb repulsion overwhelms kinetic energy. Understanding the Wigner crystal at zero magnetic field is a long-sought goal in physics, thanks to its fundamental simplicity and possible connection to the density-driven metal-insulator transition. To date, evidence for such a crystal has been reported across various platforms. However, the AC conductivity of a zero-field Wigner crystal, a key observable characterizing its electrodynamics, has never been measured. Here, we develop an ultrasensitive on-chip terahertz (THz) spectroscopy technique to probe the AC conductivity in electrostatically gated monolayer MoSe2 encapsulated in hexagonal boron nitride. We observe a sub-THz resonance corresponding to the pinning mode of a zero-field Wigner crystal, whose frequency is orders of magnitude higher than those under high magnetic fields. Using the pinning mode as an indicator, we reveal that moderate disorder notably stabilizes the Wigner crystal. With increasing density towards melting, we find that the pinning mode of the Wigner crystal coexists with a growing Drude component characteristic of an electron liquid, and the competition between these two components in the conductivity spectra leads to the insulator-metal transition of the 2D electron system. Our findings not only elucidate the low-energy electrodynamics of a zero-field Wigner crystal, but also establish on-chip THz spectroscopy as a powerful probe for correlated quantum phases in two-dimensional materials.

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arXiv2026

Supermoiré-trapped quadrupolar exciton

Moiré-trapped dipolar interlayer exciton in heterobilayers offers a rich platform to explore interaction-driven phenomena. Extending the number of layers to three and beyond leads to highly intriguing multipolar exciton - a superposition state of vertically aligned phase-coherent excitons. However, in experiments, unintentional twist-angle mismatch among layers may degrade the strength and homogeneity of the vertical Coulomb coupling. Here we propose that the supermoiré effect in a heterotrilayer comes to the rescue by creating periodic pockets of vertically aligned atomic registries that facilitate the formation of quadrupolar excitons trapped in such pockets. Using WS$_2$/WSe$_2$/WS$_2$ stack, we show interaction between multiple confined levels of the top and bottom moiré interfaces, creating electric field tunable multi-level hybridized bright (symmetric) and dark (anti-symmetric) quadrupolar states. Our work underscores the critical role of supermoiré effect in quadrupolar excitons. The discovery of reduced sensitivity on precise angle-alignment will ignite exploration of complex excitonic states in multi-layered heterostructures.

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arXiv2026

Raman spectroscopy of van der Waals topological magnet GdGaI

We report polarization-resolved Raman spectroscopy of a van der Waals compound GdGaI that is a candidate for excitonic insulators. By combining the symmetry analysis with density functional theory calculations, we identify six Raman-active phonons. The spectra exhibit only the expected anharmonic hardening down to 4 K: no additional peaks, no soft modes, and no signatures of zone folding are observed. This result indicates that any lattice distortion is below our experimental sensitivity, supporting an electronically driven origin for the band reconstruction reported by angle-resolved photoemission spectroscopy rather than an electron-phonon-driven mechanism. Moreover, we observe a pronounced circular dichroism of the $A_{1g}$ modes under an out-of-plane magnetic field. Based on symmetry considerations, we attribute this dichroic response to chiral $A_{1g}$ phonons with opposite angular momenta generated by spin-phonon coupling in the time-reversal-broken state. The temperature evolution of the degree of circular polarization further suggests that circularly polarized Raman spectroscopy detects the emergence of short-range antiferromagnetic correlations. Our results highlight GdGaI as a promising platform in which excitonic order, magnetism, and circularly polarized phonons can be intertwined, and demonstrate that circular-polarization Raman provides a sensitive probe of spin-phonon coupling in excitonic systems.

cond-mat.mtrl-sci↗
arXiv2026

Photon Correlation Spectroscopy as a Probe of Critical Fluctuations in Correlated Electron Systems

Characterizing phase transitions between correlated electronic phases, extracting their critical exponents, and identifying their universality class are of central interest in many-body physics. Here, we propose and demonstrate that photon correlation spectroscopy can be used to gain insight into the nature of critical electronic density fluctuations. We study a semiconductor moiré material consisting of two MoSe2 layers separated by a monolayer h-BN spacer and measure interlayer electron dynamics via the second-order correlation function of the scattered photons. The correlated transfer of large numbers of electrons between the layers at the onset of an Ising-type layer pseudo-spin phase transition leads to photon bunching in light scattered by the exciton resonance of one layer. Our measurements pave the way for using photon correlations as a method to access dynamical exponents associated with electronic phase transitions.

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arXiv2026

Layer-engineered quantum anomalous Hall effect in twisted rhombohedral graphene

Realizing programmable topological states in quantum anomalous Hall (QAH) insulators requires the ability to design and dynamically tune their topological invariant, the Chern number C. Here, we report a designer QAH platform based on twisted rhombohedral graphene family, in which C becomes a programmable and electrically tunable degree of freedom. By engineering the layer configuration in twisted monolayer-rhombohedral N-layer graphene, denoted as (1+N)L, we realize QAH states with C=N at moire filling v=1, where the layer number N=3,4,5 directly sets the Chern number. Beyond such static layer programming, we demonstrate in-situ electrical control. In a twisted monolayer-trilayer device, the sign of C (chirality) can be switched by electrostatic doping or displacement field. Most strikingly, in twisted Bernal bilayer-rhombohedral tetralayer graphene denoted as (2+4)L, we drive a displacement-field-induced topological phase transition between two distinct QAH states with C=3 and C=4 in a single device. Our work establishes a layer-engineered and electrically tunable platform that transitions topological quantum matter from discovery to design, opening the way toward on-demand engineering of correlated topological states and reconfigurable topological electronics.

cond-mat.mes-hall↗
arXiv2026

Link between thermodynamic correlation signatures and superconductivity in twisted trilayer graphene

Twisted graphene multilayers exhibit strong electronic correlations, which manifest in a range of experimental signatures. Yet how these signatures relate to each other and the microscopic ground states-and how twist angle and band structure reshape them-remains poorly understood. Here we study this interplay by correlating local thermodynamic and transport measurements in a twisted trilayer graphene (TTG) sample with unequal angles and flat electronic bands. We use a scanning single-electron transistor to map the impact of electron-electron interactions in a region of the sample where the local twist angle evolves smoothly. We observe gapped correlated insulators and a sawtooth in electronic compressibility, both exhibiting pronounced electron-hole (e-h) asymmetry with distinct magic angles for conduction and valence bands. Subsequent transport measurements in the same region reveal robust superconductivity with a similar e-h asymmetry. Our measurements indicate that superconductivity is not directly tied to the correlated insulators. Instead, its critical temperature correlates closely with the strength of the sawtooth in compressibility, suggesting a common origin or link between the two. By combining a local probe with transport measurements, we uncover connections between superconductivity and thermodynamic correlation signatures that are not apparent from either technique in isolation, highlighting the power of our dual approach and establishing their dependence on interlayer twist angles in TTG.

cond-mat.mes-hall↗
arXiv2026

Magnons reveal topology and dynamics of a skyrmion crystal

Although individual skyrmions are topologically protected objects, their cooperative crystalline order is fragile, easily disrupted by thermal fluctuations or other external perturbations. Probing the internal dynamics of such a crystal is both compelling and challenging, as its intricate and delicate spin texture must remain stable during measurement. Here, we engineer a nanoscale graphene junction hosting a skyrmion Wigner crystal, embedded between magnon emitters and detectors. The skyrmion crystal geometry leaves a striking imprint on magnon transport: as the gate voltage is varied, near-periodic windows of sharp fluctuations in magnon count are detected across the entire sample. We develop an interpretation that this results from skyrmions being added one by one to a quasi-one-dimensional array. Each burst of the fluctuations thus corresponds to the entry of an additional skyrmion, during which the lattice stiffness reduces. The impinging magnons induce and act as a probe of non-equilibrium collective dynamics of the crystal. These results establish a real-space probe of topological spin textures in quantum Hall-type insulating ground states via magnon transport and open opportunities to explore correlated, topologically ordered phases in moire and multilayer graphene systems.

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arXiv2026

Competing Orders Driven by Wigner Crystal Phase in Rhombohedral Graphene

Rhombohedral graphene systems provide a unique platform where strong electronic interactions and nontrivial band topology coexist at low carrier densities and high displacement fields, giving rise to a rich landscape of emergent electronic phases. Here, we report that the highly insulating state on the low-density side of chiral superconductivity in rhombohedral pentalayer graphene (R5G) corresponds to a Wigner crystal (WC) phase. In addition, a hole-doped metallic Wigner crystal (h-mWC) phase emerges near the WC boundary. Under an out-of-plane magnetic field, the system hosts competing magnetic-field-stabilized superconductivity (fSC) and unconventional reentrant quantum Hall (RIQH) states. These emergent phases are closely connected to the underlying WC and mWC states and evolve continuously across phase boundaries. Our results establish that WC phase plays an important role in the phase diagram of rhombohedral multilayer graphene and highlight its connection to a rich landscape of emergent phases.

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arXiv2026

Even-denominator fractional quantum Hall states in the zeroth Landau level of ABA trilayer graphene

Even-denominator fractional quantum Hall states (FQHSs) at half filling are of particular interest because they can host non-Abelian quasiparticles. Here we report the emergence of such states in the zeroth Landau level ($N=0$) of ABA trilayer graphene (TLG), challenging the conventional expectation that they are confined to the first excited Landau level. We observe robust incompressible states at $ν=7/2$, $9/2$, and $5/2$ with their associated Levin--Halperin daughter states: $ν=59/17$ and $46/13$ near $7/2$; $ν=58/13$ and $77/17$ near $9/2$; and $ν=43/17$ near $5/2$. These states appear exclusively within a finite displacement-field window coincident with crossings between symmetry-broken $N=0$ Landau levels carrying distinct isospin indices. The quantitative correspondence between the calculated crossing loci and the experimentally determined stability regions identifies Landau-level mixing as the microscopic origin. We attribute the stabilization of these even-denominator states to inversion-symmetry breaking in TLG, which enhances valley-resolved Landau-level hybridization and renormalizes short-range Coulomb interactions. Our results expand the landscape of even-denominator FQHSs to multilayer graphene and establish TLG as a tunable platform for realizing non-Abelian anyons.

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